This invention involves methods to recover germ fractions during or after ethanol fermentation using starch-containing corn or corn components as feedstock. By recovering germ during or after fermentation, new value-added products including germ fractions, fiber fraction and protein-enriched fraction are produced:
The corn kernel contains about 70 percent starch, 9 percent protein, 10 percent fiber, and 4 percent oil, with the rest mineral or other minor components, in three distinctive parts: (1) the pericarp; (2) the endosperm; and (3) the germ, which account for about 6 percent, 83 percent, and 11 percent of the total mass of the kernel, respectively. The pericarp is a strong fibrous seed skin, consisting primarily of coarse fiber. The endosperm consists of mainly powderous starch and gluten protein, which serve as energy reserve for seed germination and seedling growth. The germ is the embryo of the corn kernel. It consists primarily of oil and germ protein.
Corn is an important grain in US and the world as a raw material for food, feed and industrial applications. In the past decade or so, corn becomes the primary feedstock for fuel ethanol production in US. According to the Renewable Fuels Association (RFA), 22.5 percent of the total US corn crop (equivalent to about 3.0 billion bushels) in 2007/08 was used to make fuel ethanol. Of the 3.0 billion bushels of corn, about 82 percent or 2.5 billion bushels were processed by the dry-grind ethanol process, with the rest by wet milling ethanol process. Though its development seems reach a plateau in recent years (around year 2009), corn-based fuel ethanol production, especially the dry-grind fuel ethanol fermentation, is a well-established industry.
The major difference between the dry-grind ethanol process and the wet milling ethanol process is that dry-grind breaks the corn kernel into flour or meals and then ferment the whole mixture without fractionation of individual components while the wet-milling process separate the major constitutes of the corn into germ, fiber, gluten protein first and only the starch fraction is needed in fermentation to produce ethanol.
In many of the prior art dry-grind processes, the corn kernels are ground into flour using a hammer mill. The starch in the flour mixture is hydrolyzed into fermentable sugars by enzymes, and subsequently converted into ethanol by yeast. The fermented mash is then distilled to recover the ethanol. After the removal of ethanol, the mash, called whole stillage is separate into two fractions by centrifugation or decanting. One is wet cake, which is a mixture of non-fermentable solids of the corn (the oil, fiber, and protein), the other is thin stillage, which consists of water, soluble, dispersable fine solids and oil. The thin stillage is concentrated into thick stillage, a syrup-like mixture, by evaporation, and then combined with the wet cake, and dried together to produce distillers dried grains with solubles, or DDGS. Majority of the DDGS is used as low-valued cattle feed due to its high fiber content. The market for DDGS is saturated.
The dry-grind ethanol processes of the prior art which do not contain a degerming step are unable to capture the germ. The complexing of the starch with oil in dry-grind ethanol processes also reduces starch fermentability.
The wet-milling ethanol processes in prior art are the further fermentation after the conventional wet milling in which corn is fractionated into four basic components: starch, protein, fiber, and germ by using a series of grinding, separation and purification steps in an aqueous system. Only the starch fraction is used in fermentation to make ethanol. Besides starch or starch-derives (including ethanol), wet milling produces gluten meal, fiber, and germs. Germ can be further processed into edible oil, which is the most valuable component from the corn. However, wet milling requires sophisticated equipment, high capital investment, and high inputs of energy and water. Usually food grade starch and its derivatives are the main products from wet milling products due to their relatively higher values. Fuel ethanol is only a side product from a typical wet-milling company. Wet mills are usually operated at large scale with total investment near or over one billion US Dollars in order to achieve commercial efficiency. The wet-milling is a stable business dominated by about 13 companies in the world.
Compared to wet-milling ethanol process, the dry-grind ethanol process is much simpler, requiring less expensive equipment, and less capital input, thus majority of the increased capacity of fuel ethanol production is from dry-grind process, and over 75 percent of the fuel ethanol is produced in this way. The dry-grind ethanol co-product, DDGS, however, is less valuable than co-products of wet milling. Increasing the profitability of the dry-grind ethanol industry without major modifications of its infrastructure remains a challenge.
One possible strategy is to recover the oil from the downstream liquid phase of the conventional dry-grind ethanol process. However, once the oil-rich germ is broken into small pieces, the oil mixes with and is diluted by the oil-lean components including fiber, endosperm proteins, and residual starch, making it difficult to be recovered. Another problem is that the oil from the conventional dry-grind process is highly degraded, usually contains high level of free fatty acid (in a range of 9 to 15 percent).
Recovering the oil in the form of intact germ as that from wet-milling within the dry-grind industry establishment is probably the only way to ensure high yield and good oil quality.
Many of the prior-art degerming processes have been proposed over the past decade. These processes can be divided into two categories, one is dry-degerming processes and the other wet-degerming processes.
In many of the prior art dry-degerming processes, the corn kernels are moistened with water to increase their moisture content. The slightly softened corn kernels are broken into the pericarp, germ, and endosperm pieces using a coarse mill. The pieces are then screened and aspirated to separate the germ from lighter pericarp and the heavier endosperm pieces. The oil content in the germ fraction from dry degerming processes is only about 20 percent compared to about 40 percent from wet milling, and less than half of the total germ is recovered. This is because the separation of germ and other components is not complete. Germ fractions from dry-degerming contain significant amounts of endosperm and other components; at the same time more than one half of the germ is lost to the endosperm fraction. The losses of starch in the germ fraction and germ in the endosperm fraction reduce both ethanol yield and oil recovery, which compromises the economy of these processes.
Many of the prior art wet-degerming processes are modifications of conventional wet-milling process. They usually involve soaking or steeping the corn in water for a prolonged time period followed by size-reduction and fractionation in liquid phase. Water helps soften the corn and acts as a suspension medium where the kernel can be broken open to release the germ without major damage. Since oil-rich germ has lower density than the slurry medium, the germ can be isolated by floatation, such as hydrocloning or centrifugation method. After germ is removed, the starch-containing germ-free fraction is usually fermented together with at least another component (fiber or gluten protein) without further concentration or purification of the starch. The steeping or soaking time can be reduced from 24-36 hours at 52° C. in conventional wet milling to less than 12 hours at 59° C. in wet-degerming processes. The oil content in the germ is about 30 percentage. This technique is known as “Quick Germ Process”. When the pericarp (coarse fiber) is also recovered before fermentation, the process is termed “Quick Germ Quick Fiber”. There are other minor modifications to these processes, such as in “Enzymatic Milling” or “E-Milling”, where the enzyme was used to replace part or all chemicals including sulfur dioxide.
Nevertheless, these prior art wet-degerming techniques have yet to achieve widespread adoption by the dry-grind corn ethanol industry. One reason is that they still need major wet-milling equipment, including steeping tanks, degermer mills and hydroclones, which are expensive for small dry-grind plants.
These prior art processes (either dry degerming or wet degerming) all involve germ separation before the fermentation started, i.e. at the front-end. They all have lower ethanol yield than the conventional dry-grind process because some starch is unavoidably lost in the germ or fiber fractions. The oil content in the germ from is lower than that from the conventional wet milling process.
Therefore there is a need to develop new degerming processes for the dry-grind ethanol industry to achieve better fractionation (higher purity components), higher processing efficiency, easier adaptability, and to help meet the increasing demand for both food and fuel from corn.
In our invented new degerming processes the germ fraction is recovered during or after ethanol fermentation. The new processes in this invention have a few significant advantages compared to prior art front-end wet-degerming processes: 1) the total ethanol yield is higher; 2) the total germ yield is higher and the recovery of the germ is easier since the fermentation “eats away” the starch between germ and coarse fiber/endosperm proteins; 3) the coarse fiber exists in larger pieces, which can be recovered more easily; and 4) since the germ is recovered during or after fermentation, it can be processed differently and it needs less expensive equipments, such as screening apparatus, aspirators, etc.
Our invention in which the germ fraction is recovered during or after fermentation is not a simple switch of different processing steps during corn refining, because the intact germ has to go through prolonged fermentation treatment, which has different physical, mechanical, biological and chemical environments compared to that in prior art. Our invention is possible only because we studied and discovered that by proper treatments, the germ can remain physical intact during the fermentation process with little or no chemical degradation to the germ oil. Part of the data is incorporated in Example 2 of the EXAMPLES section.